Remove units in parts of compiler, dynext, setup and create the cext-lib package.

`cext-lib` contains much of the contents of `dynext`, which
is no longer very widely used.

Also moved the implementation of the `mzc` executable
to a more appropriate package.

Also, used `lazy-require` consistently for dynamically
loading implementations.

original commit: 9f2755116d

pkgs/compiler-lib/compiler/commands/info.rkt

@@ -9,5 +9,4 @@
     ("test" compiler/commands/test "run tests associated with files/directories" 15)
     ("expand" compiler/commands/expand "macro-expand source" #f)
     ("distribute" compiler/commands/exe-dir "prepare executable(s) in a directory for distribution" #f)
-    ("ctool" compiler/commands/ctool "compile and link C-based extensions" #f)
     ("demodularize" compiler/demodularizer/batch "produce a whole program from a single module" #f)))

pkgs/compiler-lib/compiler/commands/make.rkt

@@ -2,7 +2,7 @@
 (require scheme/cmdline
          raco/command-name
          compiler/cm
-         "../compiler.rkt"
+         compiler/compiler
          dynext/file
          setup/parallel-build
          racket/match)

pkgs/compiler-lib/compiler/compiler-unit.rkt

@@ -0,0 +1,5 @@
+#lang racket/base
+
+(require compiler/compiler compiler/sig racket/unit)
+(provide compiler@)
+(define-unit-from-context compiler@ compiler^)

pkgs/compiler-lib/compiler/embed-unit.rkt

@@ -0,0 +1,9 @@
+#lang racket/base
+(require racket/unit
+         racket/contract
+	 "sig.rkt"
+	 compiler/embed 
+	 "embed-sig.rkt")
+
+(define-unit-from-context compiler:embed@ compiler:embed^)
+(provide compiler:embed@)

pkgs/compiler-lib/compiler/option-unit.rkt

@@ -0,0 +1,7 @@
+#lang racket/base
+
+(require racket/unit compiler/sig compiler/option)
+
+(provide compiler:option@)
+
+(define-unit-from-context compiler:option@ compiler:option^)

pkgs/compiler-lib/launcher/launcher-unit.rkt

@@ -0,0 +1,7 @@
+#lang racket/base
+
+(require racket/unit "launcher-sig.rkt" launcher/launcher)
+
+(provide launcher@)
+
+(define-unit-from-context launcher@ launcher^)

pkgs/compiler-lib/setup/option-unit.rkt

@@ -0,0 +1,6 @@
+#lang racket/base
+(require racket/unit setup/option "option-sig.rkt")
+
+(provide setup:option@ set-flag-params)
+
+(define-unit-from-context setup:option@ setup-option^)

pkgs/compiler-lib/setup/setup-unit.rkt

@@ -0,0 +1,9 @@
+#lang racket/base
+
+(require racket/unit setup/setup-core)
+
+(provide setup@)
+(define-unit setup@
+  (import)
+  (export)
+  (setup-core))